Gauss and Riemann versus elementary mathematics · Proof We must show that there exists a unique...

1777-1855 1826-1866

Gauss and Riemannversus

elementary mathematics

Christian Skau Gauss and Riemann versus elementary mathematics

Problem at the 1987 International Mathematical Olympiad:

Given that the polynomial f (x) = x2 + x + p yields primes for

x = 0, 1, 2, . . . ,[√

]. Show that it yields primes for

x = 0, 1, 2, . . . , p − 2.

Example

p = 41,

[√413

f (0) = 41, f (1) = 43, f (2) = 47, f (3) = 53.So f (4), . . . , f (39), i.e. 61, 71, 83, 97, 113, . . . , 1601 are all primes.(First observed by Euler in 1772.)

Theorem

Let p be prime, and let fp(x) = x2 + x + p. Then the followingconditions are equivalent:

1 p = 2, 3, 5, 11, 17, 41

2 fp(x) is prime for x = 0, 1, 2, . . . ,[√

3 fp(x) is prime for x = 0, 1, 2, . . . , p − 2.

4 Q(√

1− 4p) has class number one.

Gauss (”Disquisitiones Arithmeticae“, 1801) stated that Q(√

D),where D < 0, has class number one ifD = −1,−2,−3,−7,−11,−19,−43,−67,−163.

He asked if there were other imaginary quadratic fields that hadthis property. It was shown independently by Baker and Stark in1966 that Gauss’ list was complete.

p 2 3 5 11 17 41

1− 4p -7 -11 -19 -43 -67 -163

Quadratic binary forms

f (x , y) = ax2 + bxy + cy2 ; a, b, c ∈ ZDiscriminant D = b2 − 4ac.

Example

f (x , y) = x2 + y2 ; D = −4.

Equivalent form to f (x , y):

f (x , y) = f (αx + βy , γx + δy),

where αδ − βγ = 1, i.e. [α βγ δ

]∈ SL2(Z)

[α βγ δ

[αx + βyγx + δy

]∈ Z2[

[δ −β−γ α

[δX − βY−γX + αY

]∈ Z2

��

Example

f (x , y) = 13x2 − 42xy + 34y2 = f (2x − 3y ,−3x + 5y), wheref (x , y) = x2 + y2.

If f (x , y) and f (x , y) are equivalent forms, they have the samediscriminant.

Clearly f (x , y) and f (x , y) have the same set of values as x and yrun through the integers Z.

Conversely, let g(x , y) and h(x , y) be two forms with the samediscriminant, and assume that g(x , y) and h(x , y) take the sameset of values as x and y run through Z. Then g(x , y) is equivalentto h(x , y). (We assume the forms are irreducible and primitive, i.e.the g.c.d. of the coefficients is 1.)(Schering (1859), Chowla (1966), Perlis (1979))

ax2 + bxy + cy2 ∼ AX 2 + BXY + CY 2

l l(a, b, c) ∼ (A,B,C )

[α βγ δ

]∈ SL2(Z).

D = b2 − 4ac = B2 − 4AC < 0.

Associate the complex numbers

w =−b +

2a, w ′ =

−B +√

w =αw ′ + β

γw ′ + δ.

A form is called reduced if its associated complex number w lies inthe fundamental domain for the modular group SL(2,Z). A formis reduced if (a, b, c) satisfies −a < b ≤ a < c or 0 ≤ b ≤ a = c .Every form is equivalent to a unique reduced form.

��

−12−1 11

w = −b+√

Let h(D) denote the number of inequivalent forms ax2 + bxy + cy2

of discriminant D = b2 − 4ac(≡ 0 or 1( mod 4)). Gauss provedthat h(D) is finite for all D.

Conjecture

The number of negative discriminants D < 0 which have a givenclass number h is finite. In other words, h(D)→∞ as D → −∞.

h(D) = 1⇔ Q(√

D) has class number one,i.e. the integers AD in

D) ={α + β

√D∣∣α, β ∈ Q

}is a unique factorization domain ⇔ AD is a PID.

In our case, where D is of the form 1− 4n,

{a + b

∣∣∣∣∣ a, b ∈ Z with same parity

(AD is the set of solutions to x2 + cx + d = 0; c, d ∈ Z, that lie inQ(√

There are 5 imaginary quadratic fields Q(√

D) which are Euclideanw.r.t the norm N(a + β

√D) = (a + β

√D)(a− β√D) = α2−Dβ2:

D = −1,−2,−3,−7,−11.

The units of AD are ±1 if D < 0, except for

D = −1{±1,±i} and D = −3

{±1,±ρ,±ρ2

∣∣∣∣ρ =−1 +

√−3

Example

Example of a non-unique factorization domain:

A−5 ={

a + b√−5

∣∣a, b ∈ Z}⊂ Q(

√−5).

6 = 2 · 3 = (1 +√−5)(1−√−5).

G. Rabinovitch, ”Eindeutigkeit der Zerlegung inPrimzahlfaktoren in quadratischer Zahlkorpern“.Proceedings 5th Congress of Mathematicians, Cambridge1912.

Theorem

1− 4p) has class number one ⇔ fp(x) = x2 + x + p is aprime for x = 0, 1, 2, . . . , p − 2.

We will prove that if fp(x) is a prime for x = 0, 1, 2, . . . ,[√

then Q(√

1− 4p) has class number one.

We must show that there exists a unique reduced formax2 + bxy + cy2, i.e.

− a < b ≤ a < c or 0 ≤ b ≤ a = c (∗)such that the discriminant D equals 1− 4p, i.e.

D = b2 − 4ac = 1− 4p. (∗∗)Since D is odd, b must be odd, say b = 2k + 1. By (∗∗) we get(2k + 1)2 − 4ac = 4k2 + 4k + 1− 4ac = 1− 4p, and so

ac = k2 + k + p = fp(k). (∗ ∗ ∗)The inequalities (∗), together with (∗∗), yield

−1−[√

]≤ k ≤

[√p3

]. Since fp(−k) = fp(k − 1), we get by

(∗ ∗ ∗) and our assumption that ac is a prime. By (∗) we getfinally that a = 1, b = −1 and so by (∗∗), c = p. So the uniquereduced form is x2 − xy + py2. Hence the class number ofQ(√

1− 4p) is one.Christian Skau Gauss and Riemann versus elementary mathematics

The historical continuity of mathematics (Felix Klein(1894))

Mathematics develops and progresses as old problems are beingunderstood and clarified by means of new methods.Simultaneously, as a better and deeper understanding of the oldquestions is thus obtained, new problems naturally arise.

1− 1ps

= 1 +1

(p2)s+

(p3)s+ · · · (Re(s) > 1).

1− 1ps

= (1 +1

(22)s+

(23)s+ · · · )

· (1 +1

(32)s+

(33)s+ · · · )

· (1 +1

(52)s+

(53)s+ · · · ) · · · ·

i1<i2<···<ik

(pri1i1· · · prik

=∞∑

ns= ζ(s)

ζ(s) = 2s · πs−1 · sinπs

2· Γ(1− s) · ζ(1− s).

The functional equation for the Riemann zeta-function ζ(s).Christian Skau Gauss and Riemann versus elementary mathematics

ζ(s)=∏

(1− 1

)(1− 1

)· · ·

= 1− 1

2s− 1

3s− 1

6s− 1

10s− · · · =

∞∑n=1

∫ ∞1

M(x)x−s−1 dx , where M(x) =∑n≤x

µ(n) (see next page).

µ(n) =

0 if n is not square-free

1 if n is square-free with an even number of distinct prime factors

−1 if n is square-free with an odd number of distinct prime factors

10 50403020n

∞∑n=1

ns= lim

N→∞

N∑n=1

By Abel’s partial summation we get:

N∑n=1

M(n)−M(n − 1)

N−1∑n=1

M(n){f (n + 1)− f (n)}

N−1∑n=1

∫ n+1

∫ N+1

1M(x)x−s−1dx

−→N→∞

∫ ∞1

M(x)x−s−1dx

Theorem (Hadamard and de la Vallee Poussin (1896))

critical line

critical strip

The Riemann zeta function ζ(s) has nozeros on the line Re(s) = 1.

mπ(x)

xlog x

−→ 1 as x →∞,

where π(x) denotes the number ofprimes ≤ x.(The Prime Number Theorem)

The Riemann Hypothesis (Riemann (1859))

The (non-trivial) zeros of the zeta-function ζ(s) lie on the”critical“ line Re(s) = 1

Littlewood(1912)

The Riemann hypothesis is equivalent to M(x) = o(x1/2+ε) for allε > 0, i.e.

x1/2+ε−→ 0 as x →∞, where M(x) =

∑n≤x

µ(n).

Farey fractions Fn of order n:

∣∣∣∣0 < p

q≤ 1, q ≤ n

Let An = |Fn| and let Fn ={r1 < r2 < r3 < · · · < rAn = 1

1 = 1}

δ1 = r1− 1

An, δ2 = r2− 2

An, δ3 = r3− 3

An, . . . , δAn = rAn−

An(= 0).

Example

N = 5, F5 =

}, A5 = 10.

10 110

r1 r2 r3 r4 r5 r6 r7 r8 r9 r10

Theorem (Franel and Landau (1924))

The Riemann hypothesis is truem

|δ1|+ |δ2|+ · · ·+ |δAn | = o(n1/2+ε) for all ε > 0 as n→∞.

Proof of ⇑Let f : [0, 1]→ C. Then

An∑ν=1

f (rν) =∞∑

k∑j=1

)(∗)

Comment This is the key identity, where the rather irregularoperation of summing f over the Farey fractions can be expressedmore regularly using the function M and a double sum (we showthis later).Formula (∗) applied to f (x) = e2πix gives

An∑ν=1

e2πirν =∞∑

k∑j=1

e2πi jk M

)= M(n)

sincek∑

e2πi jk =

{0 if k 6= 1

1 if k = 1

Proof of ⇑ (cont.)

Set A = An. Then

M(n) =A∑ν=1

e2πirν

=A∑ν=1

e2πi[ νA

+δν]

=A∑ν=1

e2πi νA

[e2πiδν − 1

A∑ν=1

e2πi νA

|M(n)| ≤A∑ν=1

∣∣∣e2πiδν − 1∣∣∣+ 0 ≤ 2

A∑ν=1

|sin (πδν)| ≤ 2πA∑ν=1

|δν |.

With f as in the theorem

An∑ν=1

f (rν) =∞∑

k∑j=1

)(∗)

Proof:Let 0 < p ≤ q, p and q relatively prime. The term

)= · · ·

occurs on the right hand side of (∗) with the coefficient

)+ · · · =

{1 if q ≤ n

0 if q > n(∗∗)

and so we get (∗).

Proof (cont.)

M(x) =∑k≤x

µ(k) =∑

µ(k)D(x

)where

D(x) =

{1 for x ≥ 1

0 for x < 1.

By the Mobius inversion (see next page) we getD(x) =

∑k M

Set x = nq , and we get (∗∗).

Theorem

Mobius inversion Let f , g : R+ → C.

f (x) =∑n≤x

)⇔ g(x) =

∑n≤x

µ(n)f(x

)Proof:

⇒: ∑n≤x

µ(n)f(x

)=∑n≤x

µ(n)∑m≤ x

mn≤x

µ(n)g( x

)=∑r≤x

)∑n|r

)= g(x)

Proof ⇐

⇐: ∑n≤x

)=∑n≤x

∑m≤ x

µ(m)f( x

mn≤x

µ(m)f( x

)=∑r≤x

)∑m|r

)= f (x)

We have used that∑d |n

µ(d) =

{1 if n = 1

0 if n > 1.

Dirichlet’s L-function

L(s, χ) =∞∑

∏p prime

(1− χ(p)

, Re(s) > 1,

where χ : Z/qZ→ {roots of unity} is a character. L(s, χ) can beextended to a holomorphic function on C (if χ is not the trivialcharacter).

The Generalized Riemann Hypothesis (GRH)

The (non-trivial) zeros of L(s, χ) lie on the line Re(s) = 1/2.

Theorem (Dirichlet (1839))

L(1, χ) = 2πh(D)/(const.√|D|), where D = b2 − 4ac < 0 is the

discriminant of an imaginary quadratic form ax2 + bxy + cy2, andh(D) is the class number. (χ is a primitive character mod D.)

Theorem 1 (Hecke (1918))

If GRH is true, then h(D)→∞ as D → −∞.

Theorem 2 (Heilbronn (1934))

If GRH is false, then h(D)→∞ as D → −∞.

Corollary

Gauss’ conjecture about the class number for imaginary quadraticforms is true.

D. Goldfeld.Gauss’ class number problem for imaginary quadratic fields.Bulletin AMS 13 (1985), 23-37.

P. Ribenboim.Euler’s famous prime generating polynomial and the class number of imaginaryfields.L’Enseign Math. 34 (1988), 23-42.

J. Oesterle.Le probleme de Gauss sur le nombre de classes.L’Enseign Math. 34 (1988), 47-67.

J. Sandbakken.Et merkverdig problem av Euler og den endelige avklaringen av de tre klassiskeproblemer stilt av Gauss.NORMAT 38 (1990), 101-111.

H. M. Edwards.Riemann’s zeta function.Dover Publications, 1974.

W. Narkiewicz.The development of prime number theory.Springer Verlag, 2000.

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